Interlayer dielectrics (ILDs) are used to separate conductive layers, but they must also be able to provide a way for selective connection between conductive layers. In inlaid processes, vias are typically formed through the ILD to the lower layer. In order to not overetch, the lowest portion of the ILD is an etch stop layer. Various forms of silicon nitride have been found to be effective for this purpose. Examples of such silicon nitrides include silicon oxynitride, silicon-rich silicon nitride, and stoichiometric silicon nitride. One of the characteristics of silicon nitride is that its stress is selectable. Thus, it can be either tensile or compressive. The stress of the silicon nitride, especially when the underlying conductive layer is the polysilicon layer that forms gates, has found to assist in improving the mobility of transistors. Generally, a tensile stress helps the mobility of the N channel transistors, and a compressive stress helps the mobility of the P channel transistors. Thus, a choice had to be made as to which transistor type would have improved mobility and to what extent and at what cost to the performance of the other transistor type. Thus, the concept of having tensile-stress silicon nitride over the N channel transistors and compressive-stress silicon nitride over the P channel transistors has been set forth. Practical implementation, however, has been more difficult.
Thus, there is a need for overcoming one or more of the difficulties in bringing dual-stress silicon nitride into practical implementation.